I just purchased the Echo Effect plugin as an IAP, as it is on offer, but my Apple invoice I just received indicates that I paid £14.99 GBP for the Echo plugin, which was (and still is, until tomorrow) apparently on sale at $4.99 USD, also according to an email I received from WMLabs.

Is this due to currency bias? Other apps have GBP IAP prices, which are usually more than the USD price, but not this much, especially after promotion...

You should contact Apple directly, as that looks like the regular, non-sale GBP price for PSP Echo. As all App Store's use a rolling time zone system that's based on your local App Store's time zone (plus or minus a few hours), you may have made the purchase after your local App Store's cutoff.